EP3469302B1 - Method for optically measuring the welding-in depth - Google Patents

Description

The invention relates to a method for the optical measurement of the welding depth during welding by means of laser beams.

It is known to optically measure the welding depth to use optical sensors for distance measurement that work according to the principle of optical short-coherence interferometry, in which the measuring light is split into a measuring light beam, hereinafter also referred to as measurement beam for short, and a reference light beam, hereinafter also referred to as reference beam for short becomes. The measuring and reference light beams reflected back from a measuring arm and a reference arm are superimposed on one another in order to determine the desired distance information from the path differences between the measuring and reference arm.

The area of application extends to machining processes that require an exact and automated positioning of the measuring light beam to a position in the area of the interaction zone between the working laser beam and the workpiece, in particular to the vapor capillary generated by the working laser beam at its point of impact, the so-called keyhole, such as laser welding with in- line Monitoring of the welding depth to regulate it.

Known technical solutions for the precise positioning of an optical measuring light beam during laser welding are based on camera-based methods for determining the measuring beam position relative to the working laser beam. These methods are based on an indirect determination of the position of the measuring light beam on the workpiece surface, which is required for the welding depth measurement.

The optimal position of the measuring light beam relative to the machining beam for reliable welding depth measurement, however, depends on different process parameters - such as the feed speed and the material of the weld metal - and can therefore not be determined with sufficient accuracy using the indirect method for determining the position.

The DE 101 55 203 A1 describes a laser processing device with an observation device, which is designed as a short-coherence interferometer for the acquisition of surface measurement data. A measurement at the processing point can also be used to monitor and regulate the depth of focus, for example. How the measuring point, that is to say the point of impact of the measuring beam, is to be aligned relative to the processing point in order to obtain a reliable and precise measurement of the focus or keyhole depth is not described.

The DE 10 2015 012 565 B3 relates to a device and method for increasing the accuracy of an OCT measuring system for laser material processing and describes the positioning of a measuring beam generated by an optical coherence tomograph relative to the position of the laser beam during processing with the aid of a spatially resolving sensor such as a camera. In doing so, taking into account a measuring beam position on a workpiece, a relative offset between the processing beam and the measuring beam is determined from the spatially resolved information provided by the sensor. A positioning of the measuring beam relative to the vapor capillary, i.e. to the keyhole, is not described.

The DE 10 2013 015 656 B4 relates to a method for measuring the welding depth in which two measuring beams are guided through processing optics. A first measuring beam is directed onto the bottom of the keyhole in order to measure the distance to the keyhole bottom, and a second measuring beam is directed onto the surface of the component in order to measure the distance from the component surface. The welding depth can then be determined from these two distances. It is not described how the measurement beam is aligned with the position of the keyhole.

As described above, all known methods are based on an indirect determination of the position of the measuring beam on the workpiece for the welding depth measurement. In this way, however, the exact position relative to the vapor capillary, i.e. the keyhole, cannot be determined, since it is difficult to use imaging methods to detect the exact position of the keyhole in the processing area, i.e. in the area of impact of the working laser beam.

US 2016/0039045 A1 discloses a method for measuring a depth of a keyhole using short coherence interferometry.

DE 10 2014 0007 887 A1 discloses a measuring device for acquiring surface data such as a keyhole depth of a workpiece to be processed by a laser processing device using an optical coherence tomograph.

EP 1 977 850 A1 discloses a method for material processing using a high-energy processing beam and an optical coherence tomograph for determining a surface topography of a workpiece. Based on this, the object of the invention is to provide a method for optical measurement of the welding depth, which enables the measuring light beam to be precisely positioned at the position of the keyhole and thus enables a reliable measurement of the welding depth during laser processing.

This object is achieved by the method according to claim 1. Advantageous refinements and developments of the invention are described in the subclaims.

According to the invention, to measure the welding depth during welding by means of a working laser beam, a measuring light beam of a sensor system, in particular an OCT-based sensor system, is coupled into a processing beam path of the working laser beam in a laser processing head. The measuring light beam is bundled or focused on the surface of a workpiece by focusing optics of the machining beam path in order to form a measuring light spot there. The measuring light beam reflected on the surface of the workpiece in the measuring light spot is fed back to a measuring and evaluation unit of the sensor system in order to obtain information about the distance of the surface of the workpiece from any reference position, in particular from the laser processing head. In order to obtain a surface profile of the workpiece in the area of the vapor capillary, the position of the measuring light spot on the surface of the workpiece is guided over the vapor capillary both in the welding direction and across it. The position of the steam capillary relative to the point of impact of the working laser beam is determined from the surface profile of the workpiece in the area of the steam capillary. In a subsequent laser machining process, the measuring light spot is then moved into the determined position of the vapor capillary to measure the welding depth, so that the measurement light beam is precisely aligned with the vapor capillary, i.e. the keyhole, whereby a reliable and precise measurement of the keyhole depth and thus the welding depth is ensured.

In an advantageous embodiment it is provided that the lowest point of the steam capillary is determined as the position of the steam capillary relative to the point of impact of the working laser beam. This further increases the accuracy of the welding depth measurement, since the depth of the vapor capillary, i.e. the keyhole, essentially corresponds to the depth of the weld pool in the interaction zone between the working laser beam and the workpiece.

It is expediently provided that the measuring light spot is guided on linear paths over the vapor capillary, the surface profile being determined from the measurement data along the linear paths by adapting the curve. This results in a particularly simple and quick determination.

In particular, the surface profile is determined from the measurement data along the linear path transverse to the welding direction by curve fitting according to a Gaussian distribution, while the surface profile is determined from the measurement data along the linear path in the welding direction by curve fitting according to a Maxwell-Boltzmann distribution.

An alternative embodiment of the invention is characterized in that the measuring light spot is guided on spiral paths over the vapor capillary in order to then determine the optimal measuring spot position from the distance measuring data. In principle, it is also possible to determine the welding depth along a weld seam from the surface profiles during the welding process. However, this would lead to discontinuous monitoring of the welding depth along a weld seam. It is therefore provided according to the invention that the position of the vapor capillary relative to the point of impact of the working laser beam for predetermined process parameters of a machining process is determined during a test machining and is stored as the measurement spot position for this machining process. In this way it is achieved that in machining processes that are classified by the process parameters, the measuring spot position for has been determined, the welding depth can be monitored quasi-continuously and readjusted if necessary. Thus, by readjusting the welding depth, the present invention enables not only high-quality laser processing, in particular welds, but also reliable documentation of the welding depth over the entire weld seam for quality control and assurance.

The respective position of the vapor capillary relative to the point of impact of the working laser beam is determined for specified process parameters of various machining processes during test machining and is stored as measuring spot positions for these machining processes.

According to the invention, the ideal measuring beam position is measured for various processes and with subsequent storage of the positions, e.g. directly in the sensor system. This means that welds with different process parameters can now be carried out one after the other on a system, with the previously determined positions being approached by a corresponding actuator.

In an advantageous development of the invention, it is provided that for a machining process in which the feed direction changes along the machining process, the measurement spot position stored for the corresponding process parameters is adapted to the respective feed direction. In a welding process in which the feed direction changes along the course of the weld seam, a corresponding actuator system can adapt the previously determined and stored ideal positions for the measuring light beam for the welding depth measurement to the feed direction.

The method according to the invention is expediently carried out with a device for measuring the welding depth, in particular during welding, drilling or ablation by means of a working laser beam, which has the following: a laser processing head through which a processing beam path with focusing optics is guided to focus the working laser beam on a workpiece, a sensor system for generating a measuring light beam which can be coupled into the machining beam path in the laser machining head and which is bundled or bundled into a measuring light spot on a surface of the workpiece by the focusing optics of the machining beam path can be focused, and an actuator which has a deflection unit for the measuring light beam. The sensor system and the actuator are configured in such a way that they can carry out a method according to the invention for measuring the welding depth.

• Figur 1 eine vereinfachte schematische Darstellung einer Vorrichtung zur Messung der Einschweißtiefe gemäß der vorliegenden Erfindung;
• Figur 2 eine vereinfachte schematische Darstellung eines Laserbearbeitungskopfes mit einem optischen System zum Einkoppeln eines Messlichtstrahls von einem Sensorsystem zur Einschweißtiefenmessung;
• Figur 3 eine schematische Schnittansicht eines Werkstücks zur Darstellung einer Dampfkapillare (keyhole) beim Schweißen;
• Figur 4 eine schematische Draufsicht auf die Oberfläche eines Werkstücks im Bereich einer Wechselwirkungszone zwischen Arbeitslaserstrahl und Werkstück zur Darstellung linearer Abtastlinien zur Erfassung eines Oberflächenprofils;
• Figur 5a Abstandsmessdaten einer Schweißfahrt auf einer linearen Bahn quer zur Schweißrichtung;
• Figur 5b Abstandsmessdaten einer Schweißfahrt auf einer linearen Bahn in Schweißrichtung; und
• Figur 6 eine schematische Draufsicht auf die Oberfläche eines Werkstücks im Bereich einer Wechselwirkungszone zwischen Arbeitslaserstrahl und Werkstück zur Darstellung spiralförmiger Abtastlinien zur Erfassung eines Oberflächenprofils.

The invention is explained in more detail below, for example with reference to the drawing. Show it:

• Figure 1 a simplified schematic representation of a device for measuring the welding depth according to the present invention;
• Figure 2 a simplified schematic representation of a laser processing head with an optical system for coupling in a measuring light beam from a sensor system for welding depth measurement;
• Figure 3 a schematic sectional view of a workpiece to show a vapor capillary (keyhole) during welding;
• Figure 4 a schematic plan view of the surface of a workpiece in the area of an interaction zone between the working laser beam and the workpiece to show linear scan lines for detecting a surface profile;
• Figure 5a Distance measurement data of a welding run on a linear path transverse to the welding direction;
• Figure 5b Distance measurement data of a welding run on a linear path in the welding direction; and
• Figure 6 a schematic plan view of the surface of a workpiece in the area of an interaction zone between the working laser beam and the workpiece to illustrate spiral scan lines for detecting a surface profile.

In the various figures of the drawing, components and elements that correspond to one another are provided with the same reference symbols.

As in Figure 1 As shown, the device for measuring the welding depth comprises a sensor system 10 which operates on the principle of optical coherence tomography, which makes use of the coherence properties of light with the aid of an interferometer. The sensor system 10 comprises a measurement and evaluation unit 12 which has a broadband light source (superluminescent diode, SLD; swept source light sources (spectrally tunable light sources) or the like), the measurement light of which is coupled into an optical waveguide 14. In a beam splitter 16, which preferably has a fiber coupler, the measuring light is split in a reference arm 18 and a measuring arm 20, which comprises an optical waveguide 22 and a measuring light beam path 24 which runs through a laser processing head 26. The measuring light beam path 24 comprises an optical system for coupling a measuring light beam 28 into a processing beam path 30 in the laser processing head 26. The optical system for coupling the measuring light beam 28 into the processing beam path 30 comprises, as in particular in FIG Figure 2 What can be seen is a collimator optics 32 which collimates the measuring light beam 28 emerging from the optical waveguide 22 so that it can be coupled into the processing beam path 30 in the laser processing head 26 via a partially transparent mirror 34 and superimposed on a working laser beam 36. The working laser beam 36, which is fed to the laser processing head 26 via a corresponding optical waveguide 38, for example, is collimated by collimator optics 40 and directed via the partially transparent mirror 34 to focusing optics 42, which bundle the working laser beam 36 together with the measuring light beam 28 onto the surface of a workpiece 44 or focused. To protect the focusing optics 42 from splashes and the like from the interaction zone between the working laser beam 36 and the workpiece 44, a protective glass 46 is arranged between the focusing optics 42 and the workpiece 44.

In order to be able to guide the measuring light beam 28 and thus the measuring light spot generated by the measuring light beam 28 on the surface of the workpiece 44 both in the welding direction and across the surface of the workpiece, an actuator with a deflection unit 48 is provided which divides the measuring light beam 28 into two intersecting ones Directions, e.g. B. can move in the x and y directions over the surface of the workpiece in order to scan a surface contour of the workpiece 44 and detect a corresponding surface profile. The deflection unit 48 can be used as a galvano scanner with two mutually essentially perpendicular scanning directions with reflective optics or with transmissive optics, for example Be formed prisms. Another possible embodiment of the deflection unit 48 is a device with optics that can be displaced in two directions. The deflection unit 48 is controlled by a control unit 50 in such a way that it moves the measuring light beam 28 over the surface of the workpiece 44 to detect a surface profile during a test or measuring welding run, or during a production welding process, the measuring light beam 38 and thus the measuring light spot on the one for the process parameters Keyhole position determined during the welding process. The control unit 50 can be provided as an independent unit which is connected to the sensor system 10, in particular to its measuring and evaluation unit 12, as indicated schematically by the inputs and outputs A of the measuring and evaluation unit 12 and control unit 50, or in the sensor system 10 can be integrated.

As in Figures 3 and 4 As shown, the interaction zone between the working laser beam 36 and the workpiece 44 has an area of liquid melt 52, i.e. a weld pool, which surrounds a vapor capillary 54 which is located in the impingement area 56 of the working laser beam 36 on the workpiece 44. As in Figure 3 As can be seen, the depth of the steam capillary 54 essentially corresponds to the depth of the weld pool, except for a correction factor. In the feed direction (feed direction V in Figure 3 ; x-direction in Figure 4 ) Behind the interaction zone between the laser beam 36 and the workpiece 44 is then the solidified melt 58 of the finished weld seam.

In order for a certain laser machining process, which can be classified by its process parameters, such as feed speed, laser power, focus position of the working laser beam 36 in the z-direction, i.e. in the direction perpendicular to the workpiece surface, the material of the weld metal, i.e. the workpiece 44 and / or the seam geometry, the To determine the relative position of the vapor capillary 54 to the impingement area 56 of the working laser beam 36, the position of the measuring light beam 28, i.e. the measuring light spot generated thereby on the workpiece 44 during the welding process on a linear path 60, is determined during a test or measuring welding run with the aid of the deflection unit 48, 62 moved both in the welding direction and perpendicular thereto over the vapor capillary 54, that is to say over the keyhole.

In this case, distance data along the scanning paths are recorded with the aid of the sensor system 10. For this purpose, the measuring light beam 28, which is in the processing beam path 30 is coupled, bundled or focused by the focusing optics 42 into a measuring light spot on the surface of a work back 44 and guided over the surface of the workpiece 44 by the deflection unit 48 in accordance with the selected scanning paths. The measuring light beam 28 reflected on the surface of the workpiece 44 is superimposed in the beam splitter 16, which includes a fiber coupler, with the reference light beam from the reference arm 18 and fed back to the measuring and evaluation unit 20, which is based on the information about the path differences in the reference arm 18 and measuring arm 20 information about the distance of the surface of the workpiece 44 from any reference position above the workpiece 44, for example from the position of the laser processing head 26 or the position of the focusing optics 42 therein. In order to determine the optimal positioning of the measuring light spot when measuring the welding depth, the in Figures 5a and 5b The course of the surface contour of the workpiece 44 in the area of the interaction zone between the working laser beam 36 and the workpiece 44 along the paths 60, 62 is determined by corresponding curve adaptation as point clouds along the respective paths.

The surface profile of the workpiece 44 along the path 62, that in the area of the vapor capillary 54, the depth profile of which is perpendicular to the feed direction V, is symmetrical. To determine the position of the lowest point of the vapor capillary 54, which represents the ideal position for the measuring light spot, i.e. for the point of impact of the measuring light beam 28 on the workpiece 44, the distance data are used to create a symmetrical curve for curve adaptation. The curve adaptation can expediently be carried out by means of a Gaussian distribution. $$f\left(y\right)=\frac{1}{\sigma \sqrt{2\pi }}{e}^{-\frac{1}{2}{\left(\left(y-\mu \right)/\sigma \right)}^2}$$

Here y is the position of the measuring light spot on the workpiece in the y direction, i.e. perpendicular to the feed direction V (see Figure 3 ), where µ is the expected value and σ ^{2 is} the variance of the distribution.

The depth profile of the steam capillary 54 in the feed direction is as in FIG Figure 5b can be seen, asymmetrical and corresponds roughly to the course of a Maxwell-Boltzmann distribution. $$f\left(x\right)=k1\ast x^2\ast e^{-k2\ast x^2}$$

Where x reflects the position of the measuring beam 28 on the workpiece surface in the feed direction and k1 and k2 are parameters of the distribution. The maximum of the distribution can be determined from the parameters.

Thus, the position of the lowest point of the vapor capillary 54 relative to the impingement area 56 of the working laser beam 36 is known, so that the subsequent machining of workpieces 44 with a laser machining process, which is carried out with the same process parameters as the measurement welding run, the measurement light beam 28 exactly into the keyhole, i.e. can be directed into the vapor capillary 54 in order to enable a reliable and accurate measurement of the welding depth.

The position of the keyhole 54 is stored relative to the impingement area 56 of the working laser beam 36 together with the associated process parameters. This process, i.e. the determination of the position of the vapor capillary 54, is always carried out when a welding process with process parameters is to be carried out with the laser processing head 26 for which an optimal positioning of the measuring light spot relative to the vapor capillary 54 has not yet been determined. The position of the keyhole is saved together with the process parameters each time, so that over time the positions of the keyhole are known for a large number of different laser machining processes, so that a new test weld is required when switching from one laser machining process to another when this laser machining process has never been performed by the laser machining head.

Depending on the scanner optics used in the deflection unit 48, it may be necessary that the determination of the position of the steam capillary has to be carried out repeatedly, even if this has already been carried out and stored with the given welding parameters. In particular, external disturbances such as temperature changes can lead to a Drift, that is to say to a change in the scanning position of the deflection unit 48, that is to say the position of the measuring light spot, so that the measuring light beam 28 no longer strikes the vapor capillary 54. Because of this drift, it may be necessary to repeatedly determine the position of the steam capillary 54 at specific time intervals, for example once a day or once a week.

The positions of the vapor capillary for the various laser machining processes are advantageously stored in a memory integrated in the control unit 50 or in a memory in the sensor system 10. The control unit 50 can also be an integral part of the sensor system 10; This means that welds with different process parameters can be carried out one after the other on a system, the measuring light spot, that is to say the measuring light beam 28, being set to the previously determined positions of the vapor capillary 54 by the deflection unit 48.

If the feed direction changes along the course of the weld seam during a welding process, i.e. if the feed direction deviates from the original feed direction assumed as the x-direction, the previously determined and saved ideal positions for the measuring light spot are adapted to the changed feed direction.

Instead of determining the position of the steam capillary 54, i.e. the position of the lowest point of the steam capillary 54 with the aid of distance data obtained along two intersecting linear paths 60 and 62, it is also possible to measure the measuring light spot on a spiral path 64 in FIG To guide the interaction zone between the working laser beam 36 and the workpiece 44 in order to determine the position of the vapor capillary 54 relative to the impingement area 56 of the working laser beam 36. Here, along a relatively wide spiral-shaped path 64, the approximate position of the vapor capillary 54 can first be determined from a three-dimensional funnel-shaped surface or depth profile, in order to then determine the surface or depth profile in a second measuring step by means of a narrow spiral-shaped path in the area of the impact area 56 of the To determine the working laser beam 36, from which the exact position of the vapor capillary 54 can then be determined.

Furthermore, it is also conceivable to record the surface of the workpiece 44 in the interaction zone between the working laser beam 36 and the workpiece 44 in the form of a line, the lines or paths being shifted across the workpiece perpendicular to their longitudinal extension in order to derive the to determine the exact location of the keyhole.

Claims (10)

1. Process for measuring the welding depth during welding by means of a working laser beam (36), in which process

   - a measuring light beam (28) from a sensor system (10) is coupled into a machining beam path (30) of the working laser beam (36) in a laser machining head (26) and is bundled or focused by focusing optics (42) of the machining beam path (30) into a measuring light spot on a surface of a workpiece (44),

   - the measuring light beam (28) reflected on the surface of the workpiece (44) is returned to a measuring and evaluation unit (12) of the sensor system (10) so that information is obtained about the distance between the surface of the workpiece (44) and the laser machining head (26),

   - the position of the measuring light spot on the surface of the workpiece (44) is guided over the vapor capillary (54) both in the welding direction and transversely thereto so that a surface profile of the workpiece (44) in the region of the vapor capillary (54) is obtained,

   - the position of the vapor capillary (54) relative to the point of incidence (56) of the working laser beam (36) is determined from the surface profile of the workpiece in the region of the vapor capillary (54), and

   - the measuring light spot for measuring the welding depth during laser machining is moved into the determined position of the vapor capillary (54),

   characterized in that
   the position of the vapor capillary (54) relative to the point of incidence (56) of the working laser beam (36) is determined for predetermined process parameters of a machining process during a machining test and is stored as the measuring spot position for this machining process.
2. Process according to claim 1, characterized in that the lowest point of the vapor capillary (54) is determined as the position of the vapor capillary (54) relative to the point of incidence (45) of the working laser beam (36).
3. Process according to claim 1 or 2, characterized in that the measuring light spot is guided on linear paths (60, 62) over the vapor capillary (54).
4. Process according to claim 3, characterized in that the surface profile is determined from the measurement data along the linear paths (60, 62) by curve fitting.
5. Process according to claim 4, characterized in that the surface profile is determined from the measurement data along the linear path (62) transversely to the welding direction by curve fitting according to a Gaussian distribution.
6. Process according to claim 4 or 5, characterized in that the surface profile is determined from the measurement data along the linear path (60) in the welding direction by curve fitting according to a Maxwell-Boltzmann distribution.
7. Process according to claim 1 or 2, characterized in that the measuring light spot is guided on spiral paths (64) over the vapor capillary (54).
8. Process according to any of the preceding claims, characterized in that the respective positions of the vapor capillary (54) relative to the point of incidence (56) of the working laser beam (36) are determined for predetermined process parameters of various machining processes during machining tests and are stored as the measuring spot positions for these machining processes.
9. Process according to any of the preceding claims, characterized in that, for a machining process in which the feed direction changes along the course of machining, the measuring spot position stored for the corresponding process parameters is adapted to the relevant feed direction.
10. Device for measuring the welding depth during welding by means of a working laser beam (36), comprising

    - a laser machining head (26) having focusing optics (42) for focusing the working laser beam (36), by means of which head a machining beam path (30) having the focusing optics (42) is guided onto a workpiece (44),

    - a sensor system (10) for generating a measuring light beam (28), and an optical system for coupling the measuring light beam (28) into the machining beam path (30) in the laser machining head (26), wherein the measuring light beam (28) can be bundled or focused into a measuring light spot on a surface of the workpiece (44) by the focusing optics (42) of the machining beam path (30), and

    - an actuating system which has a deflection unit (48) for the measuring light beam (28),

    - wherein the sensor system (10) and the actuating system are configured to carry out a process for measuring the welding depth according to any of the preceding claims.

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